The present invention relates generally to improved methods of severance of amorphous alloy ribbons into parts. More particularly, it relates to the cutting or parting of ribbons formed by rapid solidification of alloys having desirable sets of magnetic and related properties.
It is known that several working or forming or similar treating operations performed on amorphous alloys such as cutting or slitting, are operations which are difficult to perform when the material is worked at room temperature. The cutting of any material requires a flow of the material as the material is cut or worked. At low temperatures, the flow of amorphous alloys is governed by an inhomogeneous deformation mechanism. This deformation mechanism is characterized by high stresses. Because of the high stresses, the tools used in the ambient temperature cutting operations have short useful lives. In addition, it is known that inhomogeneous deformation of amorphous alloys is detrimental to the soft magnetic properties of the alloys.
It has been known, heretofore, that some of the difficulties experienced in working amorphous alloys can be overcome or reduced by performing the working operations at elevated temperatures. This has been reported by Masumoto in Japanese patent application No. 132288, dated Nov. 5, 1976. In this publication, it is taught that forming processes should be applied to the amorphous alloy only at temperatures above the "ductile transition temperature" and this temperature is designated as T.sub.p. The same temperature which has been regarded as critical for working has also been referred to as the "plastic transition temperature" in an article by Liebermann, in Mat.Sci.Eng. 46, 241 (1980). It is known that above this plastic transition temperature the amorphous alloys can be worked at low stresses to a high degree of straining. Patterson et al reported the hot forming of a metallic glass and demonstrated this hot forming by drawing a cup from a ribbon of amorphous alloy. This is reported in J. Patterson, A. L. Greer, J. A. Leake and D. R. H. in "Proceedings Third International Conference on Rapidly Quenched Metals," (Chameleon Press, 1978) p. 293.
More recently, Homer and Eberhardt produced strains approaching 1000% in an amorphous alloy ribbon of PdFeSi at stresses as low as 150 Mpa by a deformation which was carried out at high temperatures. This was reported in Scripta Met. 14, 1331 (1980).
In none of the foregoing studies, and methods developed from the studies, was there any concern with the effect of the rate of heating of the article to be worked on the working of the article. A primary consideration in this prior art work was the consideration of the crystallization kinetics of the alloy. An object was to effect the working without imparting significant degrees of crystallinity to the product and in this way to retain the amorphous character of the article which was being worked. The avoidance of crystallization is a primary consideration in retaining the desirable properties of the amorphous alloys.
We have succeeded in discovering a relationship between the softening and the increase in severability of an amorphous alloy article and the heating rate or the rate at which the article is undergoing heating. We have discovered that if the midsection of a length of an amorphous ribbon is heated to and maintained at a softening temperature while held in an Instron Tensile Testing apparatus and the Instron is then activated to apply tensile force to the ribbon, there is a surprising increase in tensile force needed to stretch the ribbon. In one case, a specimen length of amorphous ribbon was mounted in an Instron and the midsection of the ribbon was heated at 128.degree. C./min. to 515.degree. C. and then held at constant temperature. The Instron crosshead motion was initiated at 0.100 inches/min. and it was found that the force needed to stretch a ribbon increased from 4 MPa to 50 MPa within 30 seconds.
It is important to distinguish between the heating history of the article, that is the heating to a certain working temperature, or the rate at which an article has been heated to a certain temperature, and the effect which we have found to be important to severance of the article, namely the rate at which an article is being heated at the time the severance of the article takes place. We have found that an article such as an amorphous alloy undergoes a softening which facilitates severance when and, more specifically, during the time when it is undergoing the heating at a relatively high heating rate.
It has been observed that the heating of an amorphous strip in a narrow region such as a seam causes buckling and distortion of the strip even though the heating is not sufficient to melt or even soften the material of the narrow region.